Over the past few decades, fiber optic systems have become the standard for long-distance communication. This preponderance stems from several advantages of optical links over the more traditional, metallic-based counterparts. These include lower loss, higher information capacity, low cost per channel, immunity to crosstalk and electrical interference, and a smaller physical mass. Currently, optical fiber systems carry hundreds of terabits per second over distances &gt;1000 km. Even though this is orders of magnitude beyond the capability of metallic links, the demands of global communication are driving the system capacity to double every year.
A single-channel, fiber optic communication link necessarily includes a transmitter, optical fiber, and receiver. The transmitter converts electrical information to an optical signal by means of a modulated optical source, such as a laser. The laser can be directly modulated, or the information can be imparted to the continuous output of the laser by an external modulator. Light from the transmitter is then sent along the optical fiber to be ultimately detected by the receiver, a semiconductor which converts the optical signal back to the original electronic one.
Where the distance of communication is great, amplifiers must be included at some point in the link to strengthen the signal. In the early days of optical fiber communication, this required prematurely converting the signal to an electrical one, electrically amplifying the information, then retransmitting the amplified signal along the fiber. In order to achieve trans-continental distances, many such costly electrical amplifications were required. Moreover, because of the incessant demand for bandwidth, the single-channel links were forced to higher and higher bit rates. This basically involved modulating the optical signal more rapidly.
With the invention of the erbium-doped fiber amplifier (EDFA), the nature of the optical fiber link drastically changed. First, amplification could be performed optically, independent of data transmission format and without the need for signal conversion. Another important consequence of EDFAs was the possibility of equal link gain for a significant range of optical transmission wavelengths.
This invention has caused wavelength-division multiplexing (WDM) to become the prevalent transmission format around the globe. This format is conceptually equivalent to the use of several single-channel transmitters and receivers at various wavelengths on a single optical fiber link. In this manner, the capacity of the link is only limited by the total optical bandwidth of the amplifier (or the fiber, were the amplifier improved), and the minimum optical bandwidth separating adjacent channels.
Unfortunately, today's communications demands have strained even current high-capacity WDM links, and research is currently aimed at increasing the bandwidth of the EDFA. Typical EDFAs have approximately 32 nm of conventional bandwidth (1530-1562 nm), and a recent result of research is the long-band amplifier, from .about.1570-1610 nm. Beyond this improvement, however, little may be achieved due to the physics underlying the optical transitions of the Er.sup.+3 dopant. As a result, new materials will be required to move to shorter ranges, such as 1500 nm and below. Other rare-earth dopants, including holmium and praseodymium, have been investigated for use as optical amplifiers, but their success in providing gain over a large bandwidth has been limited.
One possibility for an optical amplifier which provides gain over a large bandwidth is Raman amplification, since it could provide up to 300 nm of bandwidth. Raman amplification, however, generally requires large pump powers (&gt;1W for a fiber &lt;100 m long), which poses a challenge in telecommunications systems.
Transition metals have long been used as optically active dopants in crystalline hosts because they fluoresce in the near infrared (1000-1500 nm) region, while exhibiting a correspondingly large bandwidth. For example, Cr.sup.4+ doped crystals that are capable of lasing near 1.3 .mu.m are disclosed in U.S. Pat. No. 4,987,575 to Alfano et al. Another example is titanium-doped sapphire (Ti:Al.sub.2 O.sub.3), which provides optical gain in the range of about 650-1100 nm.
Given the useful wavelength range and bandwidth of many transitions metal dopants, their application to telecommunications is straightforward. Since the primary telecommunications medium is glass-based optical fiber however, the crystalline-host transition metal technology of U.S. Pat. No. 4,987,575 is not suited for this application. While a natural extension would be the inclusion of transition metal dopants into glasses, their performance (particularly their efficiency) has unfortunately been found to degrade in amorphous hosts, where the crystal field strength is much smaller than single-crystal hosts.
Another approach has been considered by Alfano et al. in U.S. Pat. No. 5,717,517 whereby the laser-active Cr.sup.+4 (or V.sup.+3)-doped crystal is manufactured as a plurality of particles, to be dispersed in a "non-gaseous" medium. In this manner, the dopants remain laser-active within a crystalline host while the larger, surrounding medium is compatible with fiber optic technology. In order to minimize the optical losses from such a composite medium, both the particles and their index difference from the surrounding medium must be small. These requirements were recognized in U.S. Pat. No. 5,717,517, and the particle size was therefore stipulated to be between 0.05 and 500 .mu.m, while the index mismatch was specified to be lower than 0.1.
While the concept of dispersing crystalline particles in an amorphous medium is valid, this technology has several severe drawbacks, primary of which is the manufacture of the microscopic particles. Certainly the loss decreases with particle size, and the smallest particles (0.05 .mu.m) are therefore desirable. Grinding of material generally has difficulty producing particles smaller than 1 .mu.m and even the sol-gel method of producing forsterite has trouble attaining particles smaller than this size. While some techniques have attained particles on the 0.5 .mu.m scale, another order of magnitude smaller seems optimistic at best.
Even allowing for the smallest particle size of 0.05 .mu.m, a simple analysis of the scattering losses reveals another major shortcoming of this technique. Rayleigh scattering from randomly distributed particles can be calculated by ##EQU1##
Here scattered power is expressed as a ratio of the input power, and .lambda. represents the light wavelength, V the volume of the scattering particles, N the number of scattering particles, and m the ratio of particle index to surrounding medium index (i.e., the index mismatch ratio). With 0.05 .mu.m particles of Cr.sup.+4 -doped forsterite making up 25% of the overall medium for example, the loss using the above equation is &gt;10 dB/m at a wavelength of 1.3 .mu.m for index differences greater than 0.0005.
Moreover, since all olivines, including forsterite, are birefringent (meaning different axes of the crystal have different indices of refraction), such crystals can never be index-matched in all directions. In the case of forsterite, the index mismatch can therefore be 0.03, resulting in losses higher than 300 dB/m. Using the published optical constants for forsterite, the maximum achievable gain (complete population inversion) for a material with 25% crystalline particles would be only about 240 dB/m. This demonstrates that gain would not even be possible using forsterite and the technique of U.S. Pat. No. 5,717,517. The term "dB" as used herein is the standard optical definition as 10.times.log.sub.10 (P.sub.out /P.sub.in).
To overcome the shortcomings of the aforementioned materials and techniques, this invention describes a new class of materials, comprising a transition-metal doped glass in which extremely small crystals are internally nucleated. The process of internal nucleation forms a material called a glass-ceramic, where the crystals are less than 50 nm in size, uniformly distributed throughout the glass. The crystals are formed from constituent materials of the original glass melt, not by introducing new material as disclosed in U.S. Pat. No. 5,717,517. Moreover, the transition metal dopants are introduced into the entire medium, not just the crystals. The process of ceramming simply activates some of the omnipresent transition-metal dopant by forming a local crystal site into which the dopant is incorporated.
This doped, glass-ceramic material offers several advantages over the other, previously mentioned, transition-metal doped hosts. For example, because the crystalline phase within the biphasic glass-ceramic material is formed through controlled nucleation of the base glass, the crystalline phase has a smaller size and more uniform distribution than that obtained with other external preparation techniques. This minimizes light scattering loss (&lt;50 dB/m for index mismatches up to 0.01, according to the above equation).
Another object of the invention is to provide a uniformly transition-metal-doped, glass-ceramic gain medium comprising internally-nucleated crystals within the amorphous glass matrix, where the crystals have a size of less than 50 nm. The transition metal dopant is introduced into the entire constituent glass, and may be present in both phases of the glass-ceramic medium. It is active only within the crystal sites, and is capable of providing gain at wavelengths within the range of about 900 to 3000 nm.
It is an object of the invention to provide transition metal doped, glass-ceramic gain media that exhibit properties that make them useful as optical amplifiers or laser oscillators.
Another object of the invention is to provide glass-ceramic gain media that can provide optical gain across a wavelength range of 900-3000 nm.
A further object of the invention is to provide glass-ceramic gain media in the form of optical fiber, planar waveguide structures, bulk gain media, or any other elongated-core geometry.
A further object of the invention is to provide glass-ceramic gain media in the core of the optical fiber, planar waveguide structures, or bulk gain media.
A further object of the invention is to provide glass-ceramic gain media in the cladding of the optical fiber or planar waveguide structures.
A further object of the invention is to provide glass-ceramic gain media that require lower amounts of pump power than Raman amplification to produce a similar amount of gain.
A further object of the invention is to provide glass-ceramic gain media that have passive losses (i.e., scattering and parasitic absorption) lower than 200 dB/m, and smaller than the maximum achievable gain of the particular transition metal dopant within the crystalline sites.
A further object of the invention is to provide glass-ceramic gain media that amplify either continuous-wave (CW) or pulsed signal light.
A further object of the invention is to provide glass-ceramic laser gain media that produce either continuous-wave (CW) or pulsed (modelocked, Q-switched, or any combination therein) optical output radiation when configured as a laser oscillator.
Another object of the invention is to provide a uniformly transition-metal-doped, glass-ceramic gain medium comprising internally-nucleated crystalline sites within the amorphous glass matrix, where the crystalline sites have a size of less than 50 nm. The transition metal dopant is introduced into the entire constituent glass, and may be present in both phases of the glass-ceramic medium. It is active only within the crystalline site, and is capable of providing gain at wavelengths within the range of about 900 to 3000 nm.